1. Field of the Invention
The present invention relates to a process for preparing a biaxially drawn polyvinyl alcohol film coated with a macromolecular compound other than polyvinyl alcohol on at least one surface of the film. More particularly, it relates to a process for preparing a biaxially drawn polyvinyl alcohol film having high tensile strength and Young's modulus which has a coated layer of a macromolecular compound having a special function such as heat resistance, heat-sealable function and water resistance on its surface depending upon a usage which is prepared by coating an aqueous dispersion of a macromolecular compound such as homopolymers, copolymers, terpolymers, derivatives and modifiers thereof on at least one surface of a non-orientated substrate polyvinyl alcohol film and then, concurrent-biaxially drawing the film at high elongation percent and drying while releasing stress and heat-treating it.
2. Description of the Prior Arts
Various processes for uniaxially or biaxially drawing a film and heat-treating it to improve tensile strength and water resistance of a polyvinyl alcohol film have been proposed, for example, U.S. Pat. No. 3,440,316 discloses a process for preparing a film by drawing a polyvinyl alcohol film having a water content of less than 5 wt.% at a temperature higher than 120.degree. C. at an area elongation percent of 200% to 1000% and then, heat-treating at 140.degree. C. to 250.degree. C. For example, a film having a tensile strength of 13.6 Kg/mm.sup.2 and an elongation of 93% is prepared by a biaxial drawing at an area elongation percent of 650%.
However, the biaxially drawn polyvinyl alcohol films obtained by the conventional processes have relatively low tensile strength such as lower than 14 Kg/mm.sup.2 and various troubles have been found in applications of the films in food packaging field requiring high tensile strength.
The inventors have been studied a process for biaxially drawing a polyvinyl alcohol film in high elongation percent and have found that it is necessary to prepare a substantially amorphous non-drawn polyvinyl alcohol film while preventing orientation caused by excess tension or sudden drying during fabrication and drying of a polyvinyl alcohol substrate film. It is necessary to prepare a drawn film which is durable to heat-treatment of high degree by drying the film while releasing stress before heat-treatment after the drawing. It is further necessary to control the water content in the film in these steps, and it is necessary to impart certain characteristics to the drawn polyvinyl alcohol film such as water resistance, heat-resistance and heat sealable properties, depending upon the usage, by coating an aqueous dispersion of a macromolecular compound other than polyvinyl alcohol on at least one surface of the substrate film and concurrently biaxially drawing the coated film. It is necessary to select the concentration and pH of the aqueous dispersion of the macromolecular compound so that water in the aqueous dispersion on the surface of the substrate film diffuses and permeates into the substrate film and to modify the water content distribution in the film which normally is a high water content in the central portion of the film to a low water content in the surface layer of the film which is not a preferable distribution for drawing and which is caused by the drying of the substrate film. The present modifications highly effective for improving the drawing of the substrate film.
The inventors have found that a biaxially drawn polyvinyl alcohol film having a coated layer having useful function having high value on its surface which has a tensile strength of more than 25 Kg/mm.sup.2 especially more than 30 Kg/mm.sup.2 in optimum condition can be obtained by the combination of these steps.